JP7479370B2 - High-Temperature Superconducting Magnets - Google Patents

Description

This application relates to high temperature superconductor (HTS) magnets. In particular, this application relates to supplying electrical current to HTS magnets.

Superconducting materials are usually separated into "high temperature superconductors" (HTS) and "low temperature superconductors" (LTS). LTS materials are metals or metal alloys whose superconductivity is described by the BCS theory, such as Nb and NbTi. All low temperature superconductors have a critical temperature below about 30 K (above which the material is no longer superconducting, even in zero magnetic field). The behavior of HTS materials is not described by the BCS theory, and such materials may have critical temperatures above about 30 K (it should be noted, however, that it is the physical differences in superconducting operation and composition, not the critical temperature, that define HTS materials). The most commonly used HTS are "cuprate superconductors", which are ceramics of the cuprate family (compounds containing the oxide group of copper), such as BSCCO or ReBCO (where Re is a rare earth element, usually Y or Gd). Other HTS materials include iron pnictides (eg, FeAs and FeSe) and manganese diboron ( _MgB2 ).

ReBCO is typically manufactured as a tape with a structure as shown in FIG. 1. Such a tape 100 typically has a thickness of about 100 μm and comprises a substrate 101 (typically electropolished "Hastelloy" about 50 μm thick) on which a series of buffer layers, known as a buffer stack 102, with a thickness of about 0.2 μm is deposited by IBAD, magnetron sputtering, or other suitable technique. An epitaxial ReBCO-HTS layer 103 (deposited by MOCVD or other suitable technique) is placed on top of the buffer stack, which is typically 1 μm thick. A 1-2 μm silver layer 104 is deposited on the HTS layer by sputtering or other suitable technique, and a copper stabilization layer 105 (or "cladding") is deposited on the tape by electroplating or other suitable technique. This often completely covers the tape. Electrical current is typically coupled into the tape 100 through the cladding.

The substrate 101 provides the mechanical scaffolding and is fixed through the manufacturing line to allow for the growth of subsequent layers. The buffer stack 102 must provide a biaxially textured crystallization template onto which the HTS layers are grown, inhibiting chemical diffusion of elements from the substrate into the HTS that would affect its superconducting properties. The silver layer 104 must provide a low resistance interface from the ReBCO to the stabilizing layer, and the stabilizing underlayer 105 provides an alternative current path in case any portion of the ReBCO ceases to be superconducting (enters the "normal" state).

HTS magnets can be formed into coils by winding HTS tape, such as the ReBCO tape 100 described above. A common problem with such HTS magnets is where the individual tapes or cables leave the winding pack and enter the joint (i.e., electrical connection) area.

Figure 2 shows a schematic of a "conventional" electrical connection to a coil 201 with HTS tape 100. The outer windings of the coil 201 are partially pulled away from the winding pack to form "flying leads" 202. An electrical connection fixture 203 is placed on the flying leads 202 to provide electrical current to the coil 201.

In flying lead connections such as that shown in Figure 2, the HTS tape is subject to repeated movement under electromagnetic (EM) forces and thermal contraction, resulting in degradation during normal operation. Additionally, these "exposed" sections of the HTS tape are often at additional risk since there are no adjacent HTS wraps to share the current during critical current degradation. This means that these sections do not have the advantage of proximity to the main winding pack for heat and/or current dissipation.

Also, these flying lead areas are vulnerable to damage during the magnet winding and assembly process because the individual tapes are fragile and easily bent with mishandling. Furthermore, flying lead approaches often require the fabrication of expensive precision machined parts to guide and support the flying leads and move them from the winding pack to the joining fixture.

Another problem that can occur with superconducting magnets is quenching. Quenches occur when a portion of the superconducting wire or coil enters a resistive state. This can occur due to temperature or magnetic field fluctuations, or due to physical damage or defects in the superconductor (e.g., due to neutron radiation if the magnet is used in a melting reactor). Due to the high currents present in the magnet, even a small portion of the superconductor becoming resistive will cause a rapid temperature rise. As mentioned above, superconducting wires are provided with some copper stabilizer for quench protection. The copper provides an alternative path for the current when the superconductor goes into its normal state. The more copper present, the slower the temperature rises in the hot spots that form around the area of the quenched conductor.

Therefore, there is a need for HTS magnets that avoid or mitigate some or all of these problems.

The object of the present invention is to provide an HTS magnet that addresses or at least mitigates the aforementioned problems.

In a first aspect of the invention, an HTS magnet is provided. The HTS magnet includes a coil formed of nested concentric windings, each winding having an HTS material, and a conductive element having an electrical contact surface for supplying electrical current to at least a portion of the windings. The surface provides electrical contact between the conductive element and an axial end of the coil substantially around the at least one path of the windings.

Each winding may have an HTS tape and a cladding electrically connected to the HTS tape, and the electrical contacts may be provided to the cladding.

The electrical contact surface may provide electrical contact to the axial end of the coil over more than 20%, more than 50%, or more than 80% of the path of the at least one winding. The electrical contact surface may be ring-shaped.

The HTS magnet may have a plate extending across one or more of the other windings, and the conductive element may be formed integrally with or on the plate. The conductive element may protrude from a face of the plate, and the plate may further have a dielectric or resistive layer that electrically insulates the face of the plate from a portion of one or more of the other windings.

In this application, an "electrically resistive" layer means a layer that has an electrical resistance greater than the electrical resistance between the conductive element and the coil and between the turns of the coil (i.e., the radial electrical resistance of the coil). The electrically resistive layer may be thermoelectrically conductive, so that heat may be conducted more efficiently from (or to) the coil. The electrically resistive layer may or may not be a dielectric layer. A non-dielectric electrically resistive layer is preferred when the dielectric is subject to radiation damage, for example, when the coil is part of a Tokamak fusion reactor.

The HTS magnet may have an interface conductor layer extending across the one or more other windings to transfer heat and/or current from the end of the or each winding. The interface conductor layer may be brass and/or stainless steel. Other "solderable" metals, i.e. metals to which solder will adhere, may also be used to provide electrical contact. The interface conductor layer may be patterned by varying thickness, for example to form a "spider web" pattern.

The coil may have electrical insulation between the windings.

The HTS magnet may have one or more sensors and/or one or more heaters disposed between the plate and the coil.

The electrical contact surface may provide electrical contact to either the innermost or outermost winding of the coil. The electrical contact surface may provide discontinuous electrical contact across the windings. For example, if a coil is formed from two lengths of HTS tape, the electrical contact surface may act as an electrical junction connecting the tapes in series.

The HTS magnet may further include another conductive element having an electrical contact surface for receiving current from at least one other portion of the winding, the surface providing electrical contact to the axial end or another axial end of the coil substantially around at least one other path of the winding.

The electrical contact surfaces may provide electrical contact on opposing faces of the coil.

The HTS magnet may further include one or more additional coils, the or each additional coil having conductive elements providing electrical contacts on opposing faces of the coil, the coils being axially stacked and electrically connected to each other via their respective conductive elements. Adjacent axially stacked coils may be wound in opposite directions.

The HTS magnet may have two or more concentric nested coils, each having a respective conductive element, and each coil may be electrically connected to an adjacent coil by an electrical connection between the respective conductive elements of the coil. The electrical connections may be flexible to accommodate movement of the coils relative to one another. The HTS magnet may have one or more intervening supports disposed between adjacent coils to provide radial force isolation.

The HTS tapes of each of the adjacent coils may differ in one or more of the following: thickness, composition, width, and number.

In a second aspect of the invention, an HTS magnet is provided having first and second coils, each coil being composed of nested concentric windings, each winding having an HTS material, and first and second conductive elements, each conductive element providing an electrical connection between the coils. Each conductive element has a first electrical contact surface through which current travels to or from at least one portion of the winding of the first coil, and a second electrical contact surface through which current travels to or from at least one portion of the winding of the second coil. Each surface provides electrical contact between a respective conductive element and an axial end of the respective coil substantially along the path of at least a portion of the winding.

The electrical resistance of the electrical connection provided by the first conductive element divided by the electrical resistance of the electrical connection provided by the second conductive element may be greater than 1.5, greater than 3, or greater than 10. The area of the electrical contact surface of the second conductive element may be greater than the area of the electrical contact surface of the first conductive element.

The first conductive element may be positioned radially outward from the second conductive element. In this case, it is possible to position the first conductive element in an area of low magnetic field.

The first or second conductive element may include a variable resistor or a switch. The variable resistor or switch may include an HTS tape.

In a third aspect of the present invention, there is provided a tokamak having an HTS magnet as described above, the HTS magnet being configured to provide a toroidal or poloidal magnetic field.

In a fourth aspect of the invention, there is provided a method of forming a semi-sustaining current in the HTS magnet described above, the method comprising the steps of preparing each of the coils in a superconducting state, connecting a power source in parallel across the coils, and disconnecting the power source.

The second conductive element may comprise an HTS material, and the method may include switching the HTS material from a normal state to a superconducting state after the step of connecting a power source in parallel across the coil.

In a fifth aspect of the invention, a method of forming an electrical connection in an HTS magnet having a coil composed of nested concentric windings, each winding comprising an HTS material, is provided. The method includes providing a dielectric or resistive layer partially covering a face of the coil, providing a conductive plate on the dielectric or resistive layer, and forming an electrical contact between the conductive plate and an axial end of the coil substantially around the at least one path of the winding.

The method may further include providing an interfacial conductor layer between the dielectric layer or resistive layer and the coil, the interfacial conductor layer extending across one or more of the other windings and transferring heat or current away from the end of the or each winding.

In a sixth aspect of the invention, a conductive plate is provided for supplying current to an axial end of a coil formed of nested concentric windings. The conductive plate has a ring-shaped conductive element formed integrally with or provided on the plate. The conductive element has an electrical contact surface providing electrical contact between the conductive element and the coil. The conductive element further has a dielectric or electrically resistive layer on the conductive plate providing an electrical insulating barrier adjacent to the electrical contact surface.

The conductive plate may further include an interfacial conductive layer extending partially or completely across the dielectric layer or the resistive layer. The interfacial conductive layer is configured to transfer heat or current away from the end of the or each winding.

In a seventh aspect of the invention, a method is provided for manufacturing a conductive plate for supplying current to an axial end of a coil composed of nested concentric windings. The method includes the steps of providing a ring-shaped conductive element integrally formed with or provided on the plate, the conductive element having an electrical contact surface providing electrical contact between the conductive element and the coil, and curing a fiber-resin composite on the conductive plate to form a dielectric or electrically resistive layer on the conductive plate that provides an electrical insulating barrier adjacent to the electrical contact surface.

The curing step may include the steps of heating the composite to a target temperature, maintaining the composite at the target temperature for a predetermined period of time, and cooling the composite.

The heating rate may be less than 1°C/min, preferably less than 0.3°C/min. The cooling rate may be less than 1°C/min, preferably less than 0.4°C/min. The target temperature may be 180°C or greater. The predetermined time may be greater than 1 hour, preferably greater than 2 hours.

Also described is a method of forming an electrical and/or thermal connection to a copper surface, the method comprising the steps of providing a layer of silver on the copper surface and providing a layer of indium on the silver surface, whereby an electrical and/or thermal connection is formed to the indium layer. Also described is an electrical and/or thermal connection having a copper surface, a layer of silver, and a layer of indium, where the layer of silver is disposed directly between the copper surface and the layer of indium.

FIG. 1 is a schematic perspective view of a prior art HTS tape. FIG. 1 is a schematic front view of a prior art flying lead bond. FIG. 1 is a schematic front view of an HTS magnet. FIG. 1 is a schematic front view of an HTS magnet. FIG. 1 is a schematic cross-sectional view of an HTS magnet. FIG. 1 is a schematic cross-sectional view of an HTS magnet. FIG. 2 is a schematic radial cross-section of an HTS magnet with radial connections. FIG. 1 is a schematic radial cross-section of an HTS magnet having multiple "stacked" coils. FIG. 2 is a schematic cross-sectional view of another HTS magnet. FIG. 9 is a schematic cross-sectional view of the HTS magnet of FIG. 8 illustrating the current flowing through the magnet when a power supply is connected in parallel across the coils. FIG. 10 is a schematic cross-sectional view of the HTS magnet of FIGS. 8 and 9, illustrating the current flowing through the magnet when power is removed.

In this application, a solution to the aforementioned problem is proposed. An electrical connection is made to the HTS magnet coil through the axial end of the coil. Thus, current is supplied or received through the face of the coil. This form of connection protects the dense winding pack of the HTS tape and does not leave any part of the HTS tape separate from the coil. For example, the electrical connection is provided by a conductor in the shape of a ring placed on top of the face of the coil, which contacts the upturned ends of the windings around the circumference of the coil. By using this arrangement or "ring joint", the risk of failure of the magnet can be minimized both during assembly and during operation. Also, current can be supplied or extracted from the HTS coil without the need for flying leads, eliminating the need for many auxiliary parts, reducing the cost and complexity of the HTS magnet and simplifying its manufacture. Also, as will be shown below, such a connection or joint improves the properties of the HTS magnet.

Although references are made in this application to certain directions (e.g., up, down) or relative terms (e.g., top, upper, lower, etc.), it should be understood that these terms are used merely for the purpose of providing examples of the concepts described in this application. Similarly, although references are made in this disclosure to exemplary "pancake" coils, i.e., large flat coils formed with nested concentric windings, it will be understood from the discussion below that this disclosure is not limited to such coils.

Additionally, the integration of the ring joint into the larger structure (hereafter referred to as the Electrical-Thermal Interface (ETI) plate) allows the thermal connections, electrical insulation, and sensors that are often traditionally installed separately on the magnet to be provided as a single unit, simplifying the assembly process and allowing these components to be manufactured independently of the HTS coil.

Figures 3A and 3B show schematic front views of two possible embodiments of ring joints 300A, 300B for a pancake coil 301.

Coil 301 has nested concentric windings of HTS tape 100 arranged in a mostly flat fashion. The HTS tape 100 is wound "face to face" with opposing ends of the tape 100 protruding along the coil axis 303. Each complete winding corresponds to a complete revolution (rotation) of the HTS tape 100 around the coil axis 303. The start and end points of the outermost winding are coded 301A and 301B in FIG. 3A.

The ring joints 300A, 300B are formed by respective ring conductors 304A, 304B. For clarity in showing the coil windings, for example, the ring conductors 304A, 304B are shown behind the coil 301. Each ring conductor 304A, 304B has an annulus or ring constructed of a conductive material, preferably a metal such as copper. The ring conductors 200A, 200B meet at the upper and lower ends of the windings to provide an electrical connection to the coil 201. The ring conductor 300A is disposed on the outer diameter of the coil 201, while the ring conductor 200B is disposed on the inner diameter of the coil 201.

Each ring conductor 200A, 200B covers only a portion of the winding, so that current can be supplied to one end of the coil 201 and thereby circulate through the winding.

The ring conductors 300A, 300B each provide electrical contacts at different ends of the HTS tape 100, and they may be used as a pair to drive current radially from the outside to the inside of the coil 301 (or vice versa). For example, the coil 201 may be sandwiched between a pair of ring conductors 304A, 304B, and current may be supplied to one side (e.g., the top) of the coil 301 by one ring conductor 304A and flow through the windings of the coil 301 to generate a magnetic field. Current may then be received from the other side of the coil by the other ring conductor 304B.

The selection of the radial width of the ring conductors 304A, 304B is a trade-off between the number of turns between the junctions and the connection resistance. The connection resistance is reduced by spreading the ring conductors 304A, 304B and covering more turns of the coil 301. However, as a result, the magnetic field produced by the magnet per unit of current is reduced because there are fewer turns carrying the full magnet current. The reverse is also true if the radial width is reduced.

Because the ring joint can define a length on the order of the circumference of the coil, a low resistance connection is typically made using radially narrow ring conductors 304A, 304B that do not significantly reduce the magnetic field generated by the magnet. Although the ring conductors 304A, 304B in Figures 3A and 3B are shown extending slightly outwardly of the outer/inner ends of the coil 301, the shape of the ring conductors may instead be closely matched to the radial profile of the coil 301, minimizing the radial area of the coil 301 and the ring joints 300A, 300B.

3A and 3B to illustrate the features of the ring joints 300A, 300B, it is readily apparent that these types of joints may be applied to other shapes of coils, such as "D" shaped toroidal field coils, such as those used in tokamaks. In such cases, the ring joints 300A, 300B need not be annular, but may be shaped to follow the path of the coil windings. Similarly, the "ring conductors" 304A, 304B need not extend completely around the path of the coil windings, instead, they may extend only partially around the path of the coil windings. For example, for magnets with large radii and/or thick HTS tapes, it may be possible to form low resistance joints with ring conductors that extend only 20%, 50%, or 80% around the path of the windings. That is, the ring conductors may subtend an angle less than 360°. By introducing a "break" in the ring conductor (by not extending completely around the path of the windings), the formation of parasitic current loops within the ring conductor may be preferentially avoided. This is beneficial for applications such as nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI). In other applications, such as tokamaks (see below), geometrical constraints and/or the presence of other components may necessitate such a break.

The ring joint may be provided as part of a larger structure, which may be referred to as a conductor plate, or an Electrical-Thermal Interface (ETI) plate. The ETI plate is typically a composite metal/insulator/sensor plate that is attached to the end faces of the HTS coil and serves a number of functions:
- Means for making electrical connections to and/or between the HTS coils.
- A means to achieve "partial insulation" by introducing a control resistor in parallel with the HTS coil. The resistor's geometry is determined to tailor the dynamic electro-thermal behavior of the coil.
- A means of providing a thermal interface to the HTS coil for cooling.
- Introducing thin electrical insulation between the HTS coils and a means to mechanically protect the insulation from damage.
- A means to include accessories (sensors or heaters) in the HTS magnet without disturbing the HTS windings.

4 shows a cross section of an example magnet 400. The magnet has two ETI plates 400A, 400B mounted on a single pancake coil 401. In this example, the coil 401 has two lengths of HTS tape 100 wound around the magnet axis 303, one on top of the other. The tape 100 is clad in copper 101 as a "type 0 set" (as described, for example, in WO2018/078326), and each winding has two tapes. Insulation 402 is provided between the windings of the HTS tape 100 to prevent current from flowing across the face of the HTS tape. That is, current injected at one end of the HTS tape 100 is forced to circulate through the windings of the coil 401. The ETI plates 400A, 400B each have a respective ring conductor 404A, 404B that forms a ring joint at one end of the HTS tape 100. In this example, current is supplied to coil 401 through the radially innermost edge of HTS tape 100 using bottom ETI plate 400B. The current flows through a series of windings of coil 301 before being received by top ETI plate 400A at the radially outermost edge of HTS tape 100 via ring conductor 404A.

The ETI plates 400A, 400B are in electrical contact with the coil 401 only through the ring conductors 404A, 404B, but the plates themselves extend radially across the coil, forming a "base conductor" layer 405A, 405B through which current is supplied to (or received from) the ring joint and which provides a path for heat to be conducted away from the coil 301. In this example, the base conductors 405A, 405B are constructed from copper, although other conductive materials (e.g., metals) could be used. The ring conductors 404A, 404B could be integrally formed with the base conductors 405A, 405B or could be secured thereto, for example by soldering.

By forming the ring conductors 404A, 404B on (or integral with) the ETI plates 400A, 400B, they can be made extremely narrow in radial dimensions (sub-mm, if necessary) while still remaining easy to handle. This would be difficult to achieve if the ring conductors were separate pieces. The large surface area provided by the base conductors 405A, 405B allows heat to be efficiently removed from the coil 401 and allows for more flexibility in locating the electrical connections to the magnets.

The base conductors 405A, 405B in the ETI plates 400A, 400B can be constructed thin to minimize the temperature rise across them. The coils can be spaced less axially to avoid weakening the magnetic field strength. Alternatively, the heat can be easily extracted to the cooling bath at the outer or inner diameter of the coil 401, in which case the base conductors 405A, 405B need to be made thick enough to meet the temperature requirements. The ETI plates 400A, 400B can also be structured so that one or more of the faces of the coil 401 are sufficiently effectively cooled (i.e., not relying on thermal conduction at the radial ends of the coil). For example, the ETI plates 400A, 400B can have channels or pipes through which a gas or liquid coolant can flow to transfer heat away from the coil 401. Preferably, the channels or pipes can be provided on or within one or more of the base conductors 405A, 405B.

The use of ETI cooling plates 400A, 400B provides an alternative approach to methods such as using thermally conductive paste, which is a poor thermal conductor compared to solder, is difficult to apply effectively in thin layers, and complicates the manufacturing process.

The ETI plates 400A, 400B also have dielectric layers 406A, 406B that electrically insulate the ends of the HTS tape 100 from the base conductors 405A, 405B of the plates. The dielectric layers 406A, 406B are composed of a dielectric material, for example a fiberglass/resin composite, such as a "prepreg."

Additionally, the ETI plates 400A, 400B have an interface conductor layer 407A, 407B that can be soldered to the coil 401 for excellent thermal and electrical contact. This layer acts as a radial resistor and controls the thermal and electrical behavior of the coil 401. The introduction of such a "partial insulation", i.e., controlled "turn-to-turn" resistor, provides a desired balance between thermal stability and coil ramp time in the HTS coil. The interface conductor layer is constructed of a conductive material, preferably formed of brass or steel, because these materials can be soldered and have a higher electrical resistivity than copper. This allows it to be thicker and therefore more easily controlled in thickness. The introduction of such a "partial insulation", (PI), i.e., controlled "turn-to-turn" resistor, provides a desired balance between thermal stability and coil ramp time in the HTS coil. In particular, the use of a layer that extends across the coil windings eliminates the need for other forms of partial insulation and eliminates the need for a "wrap-around" layer of metal, such as stainless steel.

In some cases, the interface conductor layers 407A, 407B are bonded to the dielectric layers 406A, 406B by an adhesive. However, the adhesive must be able to withstand the soldering temperatures without being structurally weakened. Otherwise, the layers tend to delaminate during soldering. One way to solve this problem is to use a fiberglass/resin composite, such as a "prepreg," as both the dielectric and the bonding medium. For example, a composite such as "Prepreg MTC400" manufactured by "SHD Corp." can be used. By carrying out a relatively long curing process, the glass transition temperature (T _g ) of the composite can be increased above the soldering temperature of a normal coil. For example, the composite may be "post-cured" by heating to 180° C. at a rate of about 0.3° C./min, holding for 2 hours, and then cooling at a rate of 0.3° C./min. This procedure can result in a T _g of, for example, 200° C. This allows the composite to withstand most soldering processes that occur at lower temperatures. However, performing such a hardening process in-situ with the ETI plate on the coil can be difficult because the heating temperature and time can be damaging to the coil (due to the continuous degradation of the HTS tape as a function of temperature and time) and can damage or degrade any soldered connections already made.

However, it is also possible to use a non-insulated coil to enhance the effect of "turn-to-turn" resistance by blocking the alternative low resistance path between the windings through the copper cladding of the HTS tape 100 by the inclusion of insulation 402.

The thickness of the ETI plates 400A, 400B (i.e., the total thickness including the ring conductor) is typically in the range of 0.25-1.00 mm, the thickness of the dielectric layer (if present) is typically in the range of 10-100 μm, and the thickness of the interface conductive layer (if present) is typically in the range of 10-100 μm.

The solder used to join the interface layers 407, 407B to the windings of the coil 401 is typically PbSn. However, this material is highly conductive, and even a thin coating of PbSn on the interface layers 407A, 407B provides a sufficiently low resistance current path so that the current bypasses the interface layers 407A, 407B. To avoid this problem, a solder material is selected that has a high resistivity, preferably a higher resistivity than PbSn, for example a solder material that has a resistivity at least 10 times that of PbSn when the coil is used in a magnet at temperatures below the critical temperature of the ReBCO tape. For example, the solder material may be composed of PbBi, which has a resistivity about 50 times higher than PbSn. Similarly, PbBiSn can be used. The higher resistivity of the PbBi (or PbBiSn) solder coating (compared to a PbSn solder coating) means that more current will flow in the interface layers 407A, 407B.

Partially insulated ETI plates often have the advantage of being highly flexible. The turn-to-turn resistance can be controlled by the thickness and composition of the interface conductor. The shape of the interface conductor layer can be modified using lithography, such as by etching a finely divided spider web pattern, to interrupt long range radial currents, or inductive spirals that provide impedance. In either case, a balance can be obtained between charge time and thermal-electrical stability.

A thin electrical insulation between the pancake coils is preferable because it provides the necessary dielectric properties without causing a large temperature rise caused by heat flowing through it. However, many common dielectric materials (such as polyimide sheets) are soft, making them vulnerable to failure under the electromagnetic stresses experienced during magnet operation and assembly. By embedding the insulation inside the ETI plate and protecting the insulation on both sides with metal sheets, the risk of failure is minimized.

Because the ETI plate is a separate component to the coil, it can be substituted to change the coil's behavior. For example, an ETI plate with a thick interfacial conductor layer may be installed initially to allow the coil to operate safely and determine its critical current. Once the maximum operating current is determined, the ETI plate can be changed to one that provides the ability to ramp the magnet more quickly within the magnet's known characteristics.

Figure 5 shows a cross section of a magnet 500. The magnet 500 has two axially stacked magnets 400, each with a pancake coil 401A, 401B. The thermal and electrical connection between the coils 401A, 401B is made by interconnecting the base copper layers of adjacent ETI plates 402A, 402B after stacking. This can be done by pressing the magnets 400 axially (i.e. along the magnet's axis 503) and adding a resilient conductive layer 504, such as an indium layer, between the magnets 400 if necessary, or by soldering (although this requires heating the entire magnet). Also, the "NanoBond" (RTM) technique can be used, where a multi-layer foil is inserted between the ETI plates 402A, 402B, a chemical reaction is initiated in the foil, generating heat, and the foil is soldered to each of the plates.

For the thermal and/or electrical connection between adjacent ETI plates 402A, 402B to be effective, it is preferable that the surface condition of the plates is good, e.g. free of oxides. One way to achieve this is to provide the base copper layer of the ETI plates 402A, 402B with a layer (e.g. coating) of a noble metal such as silver. A layer of silver is preferred as silver has a low affinity for oxygen and is chemically comparable to indium. In this case, a high quality pressed connection is made using a flexible indium layer 504.

Additional plates 505A, 505B, such as copper plates, may be provided at either axial end of magnet 500 to provide additional cooling or electrical connections to magnet 500.

A power supply (not shown) is connected across plates 505A, 505B to provide current to magnet 500. In this example, the ring joints for each of the magnet faces are located at the radially outermost ends of the coils, while the ring joint connecting the two coils is located at the radially innermost ends of the coils. Thus, current flows radially inward through one winding of the coil, then axially between the coils, and then radially outward through the outer coil winding. With current flowing in opposite directions through each coil, coils 401A, 401B are wound in opposite directions (i.e., clockwise/counterclockwise) so that the magnetic field generated by each of magnets 400 has the same polarity, thereby allowing a sufficiently large magnetic field to be generated. For example, coils 401A, 401B are prepared similarly (i.e., wound in the same direction), and one of the coils is "flipped" relative to the other before stacking to form magnet 500. Obviously, this way you could stack another pancake coil with another ETI plate between them.

As mentioned above, the ETI plates may have channels or pipes through which gas or liquid may flow to transfer heat to and from the coils. Such an arrangement is particularly useful for cooling axially stacked magnets 400, as shown in FIG. 5. In particular, by providing cooling channels or pipes in the ETI plates 402A, 402B located between the coils, heat can be effectively transferred away from the "body" of the coils 401A, 401B.

Also, a ring conductor in the ETI plate can be used to make an electrical connection between nested/concentric pancake coils, i.e., to make a joint that carries current radially, rather than axially, as described above. For example, the pancake coil may be expanded by wrapping a second length of HTS tape around it. In this case, a ring conductor can be used to make an electrical joint between the ends of the two HTS tapes, preferably in the ETI plate. That is, the ring conductor is used to provide a joint across a discontinuity or break in the winding. An example where this is beneficial is stress reduction in deep coils (when the outer diameter divided by the inner diameter is large, e.g., greater than about 3 times). In such a situation, it is beneficial to sub-divide the coil into multiple nested coils and isolate the forces caused by each to reduce stress build-up in the winding. In this case, the radial connection between the nested coils can be made by a suitable ring conductor in the ETI plate.

Figure 6 shows a radial cross section of an HTS magnet 600. The HTS magnet 600 has two ring conductors 604A, 604B, providing a radial connection between an inner coil 601 and an outer coil 602. A mechanical support 607, such as a cylinder, is provided between the inner coil 601 and the outer coil 602 to break the radial forces between them.

Figure 7 shows a radial cross section of the HTS magnet 600. The HTS magnet 600 is capable of generating a high magnetic field that is compact, robust, and linear.

The HTS magnet 700 has radially nested stacks 701, 702, 703 of the HTS magnets 400 described above. For example, the radially outermost stack has HTS magnets 400A-F arranged axially, with adjacent magnets electrically connected via their ETI plates. The ring joints formed by the ETI plates are arranged so that current alternates between axial flow (between adjacent coils) and radial flow (around the windings of each coil), as represented in FIG. 6 by the arrows superimposed on the HTS magnets 400A-F. In the case of the HTS magnet 400 as shown with reference to FIG. 4, the coils of adjacent magnets are wound in opposite directions, maximizing the magnetic field along the magnet's axis 704. The other two nested magnet stacks 702, 703 have a similar configuration and reinforce the magnetic field generated by the outer stack 701. Radial joints/connections 705, 706 are formed between the end ETI plates of adjacent stacks, allowing current to flow from one stack to the next. In the example shown in FIG. 6, current is supplied to magnet 700 through the top ETI plate of the top HTS magnet 400A of the outer stack 701. As previously described, the current flows through stack 701 and then radially through junction 705 into the outer nested stack 702. Similarly, after flowing through this stack 702, the current flows radially into the inner nested stack 703 through junction 706. Finally, the current flows through the inner nested stack 703 and then is extracted from magnet 700 through the bottom ETI plate of the bottom HTS magnet of the inner nested stack 703.

In the example magnet 700 shown in FIG. 7, there are three stacks 701, 702, 703, each with six HTS coils, but it should be appreciated that any number of stacks and/or coils may be used. Also, the stacks may have different numbers of coils, providing additional freedom in the magnet design.

Compiling the HTS magnet 700 from many smaller HTS magnets ("coil sub-segments") provides many advantages. In particular, as shown below, the coil sub-segments overcome challenges related to stress limitations of the HTS tape, and the "grading" of the sub-segment coils by their placement within the magnet 700 allows for a more optimal magnet design.

Considering the stress limitations of HTS tapes, the maximum allowable transverse tensile strength of HTS tape 100 is typically around 10-50 MPa, which provides a practical limit to the radial depth of the windings that can be used. However, this problem can be avoided by subdividing the coil into several radial nested coils as shown in FIG. 7 and introducing mechanical supports 707, 708 between the coils. Similarly, axial forces can be isolated by subdivision along the magnet axis and introducing axial support structures (not shown).

Considering the "grading" of the sub-segment coils, in high field magnets the magnetic field vector can vary significantly depending on the radial and/or axial configuration of the magnet. In the case of HTS magnets, this means that the parameters characterizing the operation of the magnet as a superconductor, such as the critical current, also depend on the configuration. Thus, an optimal design of the magnet 700 can be obtained by grading the HTS tape 100 according to the location of the magnet. For example, to achieve maximum magnetic field and for quench magnets, it is desirable to maximize the ratio of current to critical current (gamma) (I/Ic) at all points of the magnet. Grading can be done by changing the number of HTS tapes per unit turn/winding, the tape width or thickness, or the tape type (i.e. manufacturer or HTS material used).

Often it is necessary to embed sensors (such as temperature or strain probes) in the HTS magnet for monitoring purposes. Other components, such as heaters, may be required for quench protection. The HTS coil and ancillary components are separate items, and it is desirable that in the event of a failure, the two items be manufactured separately and individually replaceable. A suitable compatible ETI plate can be manufactured to accommodate many sensors or other components that do not need to be embedded directly into the windings of the HTS coil.

Figure 8 shows a cross section of an HTS magnet 800. This magnet is similar to the HTS magnet 500 shown in Figure 5, except that adjacent ETI plates 801A, 801B each have an inner ring conductor 804A, 804B and an outer ring conductor 805A, 805B.

As shown in FIG. 9, the introduction of two ring conductors on each plate provides two alternative paths 908, 909 for the current to flow through the magnet 800 when a power supply 907 is connected to the set of coils. One path 908 is the same as that described with reference to FIG. 4, i.e. the current flows around the windings of each coil 400 via the inner ring conductors 805A, 805B. Another path 909 "short-circuits" or avoids path 908 by providing an electrical connection between the outer windings of the coils 400. In this case, instead of the current flowing through the outer winding to one of the coils 400 and passing around the other winding of the coil, it is led out of the coil via the outer ring conductor 804B. Similarly, the current flows through the outer winding of the other coil, between their axial ends, and led out of the coil via the outer ring conductor 804A, without passing around the other winding of the other coil.

The percentage of current flowing along each path 908, 909 is governed by the relative electrical resistance of the paths, which can be controlled by varying the electrical resistance of the outer and inner ring conductors 804A, 804B, 805A, 805B and/or the electrical resistance of the electrical contacts made by the ring conductors to the windings of the coil 400. By making the electrical resistance of the direct path 909 greater than the circuitous path 908, most of the current flows radially to and from the coil 400, making the longer, circuitous path 908 more significant than the shorter, direct path 909. This can be achieved, for example, by making the area of the outer ring conductors 804A, 804B smaller than the area of the inner ring conductors 805A, 805B. Even if a small amount of leakage current flows through path 909, which does not contribute to the magnetic field generated by the circulation of current through the windings of the coil 400, the coil 400 will still be charged to the full critical current in the inner winding, and the additional leakage current will be carried by the outer winding. Because they have a high critical current in this low magnetic field region. Once the magnet 800 is charged, the power supply 907 is disconnected and the current flows around the coil 400 in a closed loop.

Figure 10 shows the path 1010 of the "trapped" current flowing in magnet 800 after power supply 907 is disconnected. In this case, the current passes through the windings of coil 400, around the closed loop, and through each of ring conductors 805B, 805A, 804A, and 804B. Because coil 400 is superconducting, the current is allowed to flow around path 1010 for an extended period of time before decaying, i.e., it is a circulating current, and the resulting magnetic field is "semi-persistent."

The time constant for the decay of the circulating current is determined by the ratio (L/R) of the coil's magnetic self-inductance (L) to its electrical resistance (R). For example, considering a magnet with a coil of inner diameter 50 mm and outer diameter 98 mm, the self-inductance is ~2 mH and the electrical junctions typically have a resistance of about 1-5 nΩ under liquid nitrogen (i.e., an estimated combined resistance of the inner and outer electrical junctions of ~10 nΩ). The L/R time constant for this magnet is about 200,000 seconds, or 2.3 days.

A large time constant also correlates with a large "charging" time, i.e., the time for a steady-state distribution of current to build up between paths 908, 909 when a power source is connected. To minimize the charging time, it is beneficial to increase the resistance of path 908 during charging (i.e., the configuration shown in FIG. 9). This can be achieved by a variable resistor or switch introduced at the outer electrical junction provided by the outer ring conductors 804A, 804B. For example, an HTS switch with an HTS material may be provided between the outer ring conductors 804A, 804B. During charging, the switch is turned "off" (non-superconducting state) and provides a large resistance, which allows for rapid charging. This is achieved, for example, by heating the HTS material to a temperature above a critical temperature. The switch is then switched "on" (e.g., no further heating or cooling), closing the current path 1010 and disconnecting the power source.

One important application of HTS magnets such as those mentioned above is in a type of nuclear fusion reactor known as a tokamak. Tokamaks are characterized by a strong toroidal magnetic field, high plasma current, and a combination of a typically large plasma volume and large preheating, providing a stable plasma at high temperature. This allows them to create the conditions in which nuclear fusion can occur. Preheating (e.g., by injection of a tens of megawatts of neutron beam of high-energy hydrogen, deuterium, or tritium) is required to raise the temperature to a high enough value required for fusion to occur and/or to sustain the plasma current.

The magnet coils in a tokamak can be divided into two groups. Poloidal field coils are horizontal circular coils wound so that their centres are placed on the tokamak's central column, creating a poloidal magnetic field (i.e. substantially parallel to the central column). Toroidal field coils are wound vertically through the central column and around the outside of the plasma chamber (the "return limb"), creating a toroidal magnetic field (i.e. circular around the central column). The combination of the poloidal and toroidal fields creates a helical field within the plasma chamber, which sustains the confined plasma.

The currents required to generate the toroidal field are extremely large. Tokamak designs therefore increasingly use superconducting materials in the field coils. In small spherical tokamaks, the diameter of the central column needs to be as small as possible. This presents a conflicting requirement, since even with superconducting materials there is a limit to the current density that can be achieved.

The HTS materials described herein are particularly suitable for use in tokamaks, particularly spherical tokamaks, for example, in generating either poloidal or toroidal fields (or both).

As described above, various embodiments of the present invention have been described by way of example only, and should be understood to be illustrative and not limiting. It will be apparent to those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the present invention. For example, while the coils described above are described as having HTS tapes 100 arranged in a "Type 0" configuration, other configurations such as "Type 1" and "Type 2" may also be used (e.g., as described in International Publication WO2018/078326). Similarly, while in the example described above, the coils are generally connected in series across the power supply, the coils may also be connected in parallel across the power supply. Accordingly, the present invention is not limited to any of the embodiments described above, but is defined only by the appended claims and equivalents thereof.

Claims (19)

A high temperature superconductor (HTS) magnet,
a coil including nested concentric windings wound about an axis, each winding having an HTS material;
first and second conductive elements each having a respective electrical contact surface through which current is supplied to or received from a respective portion of at least one of the windings, each electrical contact surface providing electrical contact about at least a portion of a circumference of a respective one of the at least one of the windings between the respective conductive element and an axial end face of the coil;
first and second conductive elements, the electrical contact surface of the first conductive element being disposed closer to the axis than the electrical contact surface of the second conductive element, such that a current supplied to the coil via the first conductive element circulates through the coil before being received by the second conductive element, or a current supplied to the coil via the second conductive element circulates through the coil before being received by the first conductive element;
An HTS magnet having Each winding includes an HTS tape and a cladding electrically connected to the HTS tape;
The HTS magnet of claim 1 , wherein the electrical contact is provided to the cladding. The HTS magnet of claim 1 or 2, wherein each electrical contact surface provides electrical contact to one or more of the axial end faces of the coil over more than 20% of the path of at least one of the respective windings. 4. The HTS magnet according to claim 1, wherein the electrical contact surface of the first conductive element and/or the electrical contact surface of the second conductive element are ring-shaped. having at least one plate extending across one or more of the other windings different from the winding on which the electrical contacts are provided ;
5. The HTS magnet of claim 1, wherein the first or second conductive element is formed integrally with or on the plate. the first or second conductive element protrudes from a surface of the plate;
6. The HTS magnet of claim 5, wherein the plate further comprises a dielectric or electrically resistive layer that electrically insulates the face of the plate from one or more portions of the other windings. an interface conductor layer extending across the one or more other windings;
7. The HTS magnet of claim 6, wherein heat and/or current transfer from an axial end face of the one or more other windings. The HTS magnet of claim 7, wherein the interface conductor layer comprises brass and/or stainless steel. The HTS magnet of claim 7 or 8, wherein the electrical resistance between the windings of the coil is controlled by varying the thickness of the interface conductor layer. The HTS magnet of any one of claims 7 to 9, wherein the coil has electrical insulation between the windings. The HTS magnet according to any one of claims 5 to 10, comprising one or more sensors and/or one or more heaters disposed between the plate and the coil. The HTS magnet of any one of claims 1 to 11, wherein the electrical contact surface of the first or second conductive element provides electrical contact to either the innermost or outermost winding of the coil. The HTS magnet of any one of claims 1 to 12, wherein the electrical contact surfaces provide electrical contact to opposing faces of the coil. Further, it has an additional coil,
the additional coil having nested concentric windings wound about an axis of the additional coil;
the additional coil having first and second conductive elements providing electrical contacts on two opposing sides of the additional coil;
14. The HTS magnet of claim 1, wherein the coil and the further coil are axially stacked and electrically connected to each other via respective conductive elements. The HTS magnet of claim 14, wherein the windings of the coil and the windings of the additional coil are wound in opposite directions. the coil is divided into an outer sub- coil and an inner sub- coil, the inner sub- coil being concentrically nested within the outer sub- coil;
16. The HTS magnet according to claim 1 , wherein the windings defining the inner sub-segment coil are electrically connected to the windings defining the outer sub-segment coil by further first and second conductor elements. 17. The HTS magnet of claim 16, further comprising one or more mechanical support structures disposed between the inner and outer sub -segment coils for radial force isolation. 18. The HTS magnet of claim 16 or 17, wherein the HTS tapes of the inner sub-segment coil windings and the outer sub-segment coil windings differ in one or more of thickness, composition, width, and number. A tokamak comprising an HTS magnet according to any one of claims 1 to 18,
The HTS magnets are configured to provide a toroidal or poloidal magnetic field.

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